1. Field of the Invention
The present invention relates to a motor driver circuit and a three phase direct current brushless motor.
2. Description of the Related Art
FIG. 1 illustrates a conventional three phase brushless direct current (DC) motor driver circuit 1. Three phase coils Lu, Lv, Lw of the three phase brushless DC motor are electrically connected to each other in a Y circuit configuration, wherein terminals on one end of the three phase coils Lu, Lv, Lw are electrically connected to each other (contact n), and the terminals on the other end (contacts U, V, W) are electrically connected with the drive circuit 1. Driver circuit 1 has three bridge arms coupled in parallel with a DC power supply Vdc (hereinafter referred to as the U-phase, V-phase and W-phase bridge arms). Each bridge arm has an upper switch U+, V+, W+, a lower switch U−, V−, W− and two flywheel diodes D each electrically coupled with a respective one of the upper and lower switches in parallel. The contacts U, V, W of the three phase coils Lu, Lv, Lw are electrically connected between the upper switch U+, V+, W+ and the lower switch U−, V−, W−, of the bridge arms, respectively.
The drive circuit 1 controls, for example, upper switch U+ of the U-phase bridge arm and the lower switch V− of the V-phase bridge arm to be switched on in a basic cycle. This enables the U-phase coil Lu, and the V-phase coil Lv and the power supply Vdc (see FIG. 2), generating a magnetic force for driving a rotor of the DC brushless motor.
At the end of the basic cycle, the upper switch U+ and the lower switch V− are switched off (see FIG. 3), and back electromotive forces eu,ev are instantly produced by the phase coils Lu, Lv. A large current attributed to the back electromotive forces eu,ev will pass through the flywheel diode D coupled in parallel with the upper switch V+ and the flywheel diode D coupled in parallel with the lower switch U−, applying a high voltage surge across the power supply Vdc that may easily damage the same.
Referring to FIG. 4, a conventional solution to the abovementioned problem is to provide a dissipating resistor R controlled by a switch SW to make or break parallel connection with the power supply Vdc. At the instant the upper switch U+ and the lower switch V− are switched off, the switch SW connects the Resistor R with the power supply Vdc in parallel to achieve an effect of lowering the voltage across the power supply Vdc. This protects the power supply Vdc from the instantaneous high voltage attributed to back electromotive forces eu, ev.
Although such an approach may suppress the high voltage attributed to back electromotive forces, power of the power supply Vdc is dissipated by the dissipating resistor R in vain and consumption of power of the power supply VDC is accelerated.
Since the back electromotive forces cannot be stored by the power supply Vdc as it has an alternating current form, the energy dissipated will cause temperature to rise in the driver circuit 1, and such high temperature will shorten the service life of the power supply Vdc (usually a battery).
In addition, as compared to the delta circuit configuration, the currents in the Y circuit configuration are smaller and thus the mechanical power outputted by the motor is smaller. However, if the delta circuit configuration is used, the back electromotive forces produced will be greater than that of the Y circuit configuration, which is unfavorable for the driver circuit 1. Therefore, the conventional driver circuit 1 is not suited for the delta circuit configuration.